(1) Field of the invention
The present invention relates to an apparatus for recognizing the shape of a semiconductor wafer, capable of automatically recognizing the shape of a semiconductor wafer when the semiconductor wafer is being processed on a dicing apparatus or the like so that the dicing apparatus or the like can process and handle the semiconductor wafer accurately and efficiently according to the shape of the semiconductor wafer even if the semiconductor wafer is cracked or chipped.
(2) Description of the Related Art
After fabricating a plurality of semiconductor chips on the surface of a semiconductor wafer, the semiconductor chips are inspected with an IC tester provided with a prober and the semiconductor wafer is diced by a dicing apparatus. In the art of automatically processing the semiconductor wafer, automatization has been developed in order to align the semiconductor wafer automatically and perform various processes on an assumption that the semiconductor wafer is circular. The following explanation will be made in connection with a dicing apparatus by way of example. The dicing apparatus cuts narrow grooves in the surface of the semiconductor wafer with a cutting blade rotating at a high speed, which is widely used for cutting the semiconductor wafer into semiconductor chips.
The pattern of the semiconductor chips fabricated on the surface of the semiconductor wafer cannot be correctly positioned relative to the working position of the cutting blade by simply mounting the semiconductor wafer on the stage and hence the grooves cannot be accurately cut along desired lines on the semiconductor wafer without adjusting the position of the semiconductor wafer relative to the cutting blade. The position of the semiconductor wafer relative to the working position of the cutting blade is adjusted by moving the stage supporting the semiconductor wafer into the field of view of an image pickup device and aligning a specified position on the semiconductor wafer with a specified position in the field of view of a projector. The projector, namely the image pickup device, is located beforehand at a predetermined position relative to the working position of the cutting blade. After thus determining the positional relation between the specified position on the surface of the semiconductor wafer and the working position of the cutting blade, the stage is moved by a precision stage moving mechanism to cut grooves accurately along the desired lines on the semiconductor wafer. The operation to adjust the position of the semiconductor wafer relative to the working position of the cutting blade is called an alignment operation.
When carrying out the alignment operation, for example, an image represented by the video signals provided by the image pickup device such as a video camera is displayed on the screen of a monitor and the operator adjusts the position of the semiconductor wafer while observing the image displayed on the screen of the monitor. Recently, automation of the dicing apparatus has been desired for the enhancement of productivity and an automation of the alignment operation has been realized. The automation of the alignment operation can be achieved by image processing the video signals provided by the image pickup device.
When carrying out automatic alignment through image processing, specific points on the wafer are brought into the field of view of the image pickup device, and the boundaries between the lines and the rows of the semiconductor chips are detected. Then, the inclination and the position of the lines and rows of the semiconductor chips are determined on the basis of the results of detection, the stage is turned so that the lines and rows of the semiconductor chips are aligned with the axes of the moving mechanism, and the coordinates of the moving mechanism in a state where the specified position on the semiconductor wafer coincides with a specified position on the image formed by the image pickup device are stored in a memory. The identification of the lines and the rows of the semiconductor chips using the specific points can be achieved by a known pattern matching method or the like. Accordingly, when the specific points are brought into the field of view of the image pickup device for identification, the identification of the lines and the rows of the semiconductor chips requires a comparatively short time if the specific points are positioned near the center of the field of view of the image pickup device. However, if the specific points are located away from the center of the field of view of the image pickup device, the automatic alignment operation requires a comparatively long time because the operation for pattern matching must be repeated many times.
The semiconductor wafer has a circular or substantially circular shape. Therefore, the specific points are moved for the automatic alignment operation to move the specific points into the field of view of the image pickup device on an assumption that the semiconductor wafer is circular, the semiconductor wafer is supported on the frame with its center substantially on the center of the frame, and the direction of extension of the lines of the semiconductor chips is within a certain range relative to notches formed in the frame. The pattern matching operation is performed after the semiconductor wafer has been thus positioned.
In manufacturing processes, some semiconductor wafers are cracked or chipped, and the semiconductor wafer is not always supported on the frame with its center in coincidence with the center of the frame. The yield rate of the process will be reduced if cracked semiconductor wafers and chipped semiconductor wafers are thrown away. Therefore, defective semiconductor wafers must be diced to provide normal semiconductor chips in order to increase the yield rate. However, when adjusting the position of a cracked semiconductor wafer by an automatic alignment operation, the automatic identification of the lines and rows on the semiconductor chips is impossible because the positions for identifying the lines and rows of the semiconductor chips cannot be determined, and a part of the semiconductor wafer not provided with lines or rows of semiconductor chips may be brought into the field of view. When the center of the semiconductor wafer is located far away from the center of the frame, it is again impossible to identify the lines and rows on the semiconductor chips.
The applicant of the present patent application disclosed a dicing apparatus which starts its operation after recognizing the shape of the semiconductor wafer in Japanese Unexamined Patent Publication (Kokai) No. 4-109652. A dicing apparatus capable of recognizing the shape of the semiconductor wafer by image processing is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 4-233250. In the dicing apparatus mentioned above, an image pickup device is disposed so that specularly reflected light will not fall thereon. Since the surface of the semiconductor wafer is a highly accurate flat plane, most part of the illuminating light falling on the surface of the semiconductor wafer is specularly reflected and little reflected light falls on the image pickup device and hence a region in the image corresponding to the semiconductor wafer is very dark. Although the flatness of the surface of the frame is not as high as that of the surface of the semiconductor wafer, a region in the image corresponding to the frame is somewhat dark because the surface of the frame has a comparatively high flatness. Since the surface of the adhesive sheet reflects the illuminating light irregularly, a region in the image corresponding to the surface of the adhesive sheet is bright. Accordingly, when processing the image to recognize the shape of the semiconductor wafer, the image is converted into a digital image by converting the brightness values of the image into digital values with reference to an appropriate threshold.
Sometimes, the region in the image corresponding to the surface of the semiconductor wafer is brighter than the region of the same corresponding to the surface of the adhesive sheet when the surface of the semiconductor wafer is in a particular condition. In such a case, the image data is inverted so that the images of bright surfaces are dark and the images of dark surfaces are bright, and then, the inverted image data is processed in the foregoing manner to recognize the shape of the semiconductor wafer.
Once the shape of the semiconductor wafer is recognized, the automatic alignment operation becomes possible because the positions of the lines and the rows of the semiconductor chips can be specified, and useless machining time can be dispensed with when dicing the semiconductor wafer because unnecessary parts are also cut although only part of the semiconductor wafer needs to be cut for dicing.
Some circuit patterns formed on the surface of the semiconductor wafer interfere with the perfect specular reflection of the incident illuminating light and causes part of the reflected light to fall on the image pickup device, which is inferred to be due to the diffraction of the light. If such a phenomenon occurs, the image pickup device provides video signals representing a partly bright image of the semiconductor wafer. If such image is digitized using the same threshold as that used for digitizing the image, the semiconductor wafer is recognized as a cracked semiconductor wafer and an accurate recognition of the shape of the semiconductor wafer is impossible.
A warp in the wafer and irregularly finished surface of the wafer are other effects that make accurate shape recognition impossible.
In case the semiconductor wafer is warped or the surface of the semiconductor wafer is not uniform, the shape of the semiconductor wafer cannot be accurately recognized.
Moreover, due to the transmission rate depending on a color of the adhesive sheet, not all of incident illuminating light on the semiconductor wafer is reflected, and the generally incident illuminating light is irregularly reflected and brighter light is received by the image pickup means from some parts of the adhesive sheet because the stage is painted in white. Therefore, depending upon the transmission rate of the color of the adhesive sheet, there is a phenomenon such that irregular reflection occurs in the area where the transmission rate is high and then the image pickup means receives the reflected light, while the irregular reflection does not occur in the area where the transmission rate is low and then the image pickup means does not receive the reflected light.